The International Telecommunication Union has specified interfaces for a new transport network having the designation OTN (optical transport network) in its new recommendations ITU-T G.709 (Prepublished Recommendation, 02/01) which is incorporated by reference herein. Said new transport network serves the high-speed transmission of digital optical signals in future WDM (wavelength-division multiplex) networks. In contrast to previous transport networks, such as SDH (synchronous digital hierarchy) networks and SONET (synchronous optical network), OTN operates with substantially asynchronous optical signals. This means that the individual communications signals are not synchronized to a common network timing and may have different bit rates within a specified range (±20 ppm).
For said novel transport network, network elements now have to be developed that are capable of switching the newly specified communications signals. Said network elements are, in particular, optical crossconnects and add/drop multiplexers. The essential feature of such network elements is a switching matrix that switches communications signals from any to any port in an arbitrary controlled manner.
Since OTN is an optical transport network, the use of a purely optical switching matrix would suggest itself in network elements. Such a matrix makes it possible to switch so-called OTUk signals (optical channel transport unit, k=1, 2 or 3). Such OTUk signals are characterized by their frame structure, bit rate and bandwidth. At present, available, purely optical switching matrices require, however, a number of optical amplifiers and repeaters and are therefore technically very complex and expensive, they suffer from limited capacity and reliability and they are bulky. In addition, it will be necessary to evaluate and to terminate certain information in the transport frames of the communications signals, which inevitably requires electrical processing.
Alternatively, an asynchronous electrical switching matrix is suitable, i.e. a switching matrix that is made up of a multiplicity of electrical switching circuits which make it possible to switch electrical signals of any bit rate in a certain bandwidth range. Such integrated circuits are, however, only being developed and are at present not yet available. The high bit rates of the OTN signals (2.67 Gbit/sec, 10.71 Gbit/sec and 43.12 Gbit/sec) present the development of such synchronously operating integrated circuits with major technical problems.